Formation of bicyclic ethers from Lewis acid promoted cyclizations of cyclic oxonium ions.

Oxacycles carrying a vinyl group and an acetal were extended with a cyclization terminator (vinyl silane or vinyl sulfide) by Suzuki coupling. Treatment with Lewis acid induced cyclization to provide bicyclic ethers in reasonable yields. In the case of the vinyl silane, an ene-like mechanism is preferred, whereas the thioenol ether enters into a Prins-type reaction channel. In one instance, the bicyclic compound was opened to give the ten-membered functionalized enone 24. [reaction: see text]